Method for the wet-chemical etching of a highly doped semiconductor layer

ABSTRACT

A method for the wet-chemical etching of a silicon layer in an alkaline etching solution is provided, where the silicon layer is the surface region of a solar cell emitter. The method ensures that the surface region of the emitter is etched-back homogeneously using an oxidant-free alkaline etching solution comprising at least one organic moderator is used for the isotropic etching back of the surface region of the emitter, where the moderator has a dopant concentration of at least 10 18  atoms/cm 3 .

The invention relates to a method for the wet-chemical etching of asilicon layer in an alkaline etching solution, the silicon layer beingthe surface region of a solar cell emitter.

The quality factor/structure of an emitter is one of the crucial factorsfor the efficiency of a solar cell. The emitter depth, the dopantprofile, the surface concentration of the dopant, and the emitter layerresistance must be precisely adjusted to obtain optimal cell properties.

The emitter properties are influenced by the diffusion parameters oftemperature and time as well as the type of doping agent. In theframework of economically applicable industrial processes, however, itis not possible to adjust all properties at will and independently ofone another.

In the currently widespread methods for producing crystalline solarcells, homogeneously doped mono- and multicrystalline p-type siliconwafers, usually with boron base doping, are used as starting materials.The concentration of the dopant is on the order of 10¹⁶ atoms/cm³. Theemitter is produced by in-diffusion of phosphorus. The most importanttarget parameter, which is readily accessible by measurement technology,is the emitter layer resistance as a measure of the amount ofelectrically active phosphorus atoms that have diffused in.

The phosphorus surface concentration can lie in the range of 10¹⁹ tomore than 10²¹ atoms/cm³, depending on the diffusion conditions and thetype of doping agent, with the maximum concentration of electricallyactive phosphorus atoms as well as the solubility of phosphorus insilicon (approximately 5×10²⁰ atoms/cm³) being exceeded. The phosphorusis then present in inactive form and forms recombination centers for thecharge carrier pairs generated (so-called “dead layer”).

The metal contacts on the front side are produced predominantly by meansof thick-film silver pastes in the silk-screen printing process withsubsequent sintering. On the one hand, a high phosphorus surfaceconcentration is advantageous for the creation of a low-ohmic contactbetween the silver paste and the emitter; on the other hand, such a highsurface concentration of the doping agent causes, as mentioned, a moreenhanced recombination of the charge carrier pairs generated by lightabsorption.

An improvement of the emitter surface can be achieved by etching back ofthe highly doped layers near the surface. The surface recombination ofthe charge carriers generated is hereby reduced.

This is evident through improved spectral sensitivity and internalquantum efficiency in the short-wavelength region of the spectrum andpositively influences above all the short-circuit current density aswell as the open-circuit voltage of the solar cell.

According to the prior art, in a first chemical treatment step, thephosphosilicate glass formed from silicon oxide and from dopant oxidesis removed in dilute hydrofluoric acid after diffusion during theproduction of solar cells with p-silicon-based wafers.

In a second treatment step, alkaline cleaning solutions, based onammonia or alkyl ammonium hydroxides and hydrogen peroxide, are used.Solutions of this type were originally developed for semiconductorcleaning, known as “SC-1,” a part of the RCA cleaning sequence.

Standard RCA cleaning comprises treatment in the SC-1 solution,consisting of ammonia or ammonia derivatives and hydrogen peroxide,rinsing, and treatment with hydrogen peroxide in a dilute hydrochloricacid solution (known as “SC-2”). Ammonia and hydrochloric acid aregenerally employed in the concentration range of 3 to 10 weight percent;the hydrogen peroxide concentration generally is approximately 1 weightpercent.

The temperatures employed are in the range of 60° C. to 85° C.; thecontact times are approximately 10 minutes. Such conditions can berealized only in batch process units.

In widely employed continuous process units, solutions based on alkylammonium hydroxides and hydrogen peroxide are employed at temperaturesof approximately 60° C. and contact times of approximately 60 s. Forreasons of economic efficiency, markedly longer contact times cannot beimplemented.

The change of the emitter layer resistance is a readily accessiblemeasured parameter, by means of which the depth of the emitter etchingback and the reduction in the surface concentration of the dopant can bemeasured approximately. Mass erosion due to the etching solution is verysmall (on the order of 1 mg for wafers that are 156×156 mm in size) andcorresponds to the erosion of only a very thin emitter layer of lessthan 10 nm.

Emitters produced by standard methods have a layer resistance in therange of 50 to 70 ohm/sq after diffusion. The layer resistance is raisedby 5 to 10 ohm/sq by treatment in dilute hydrofluoric acid andsubsequent treatment in the RCA sequence, that is, 10 minutes in SC-1solution, rinsing, and 10 minutes in SC-2 solution. The emitter etchingback takes place predominantly in the alkaline SC-1 solution.

The layer resistance is increased by only approximately 1-2 ohm/sq bytreatment in continuous process units at approximately 60° C. and acontact time of 60 s in a solution based on alkyl ammonium hydroxidesand hydrogen peroxide.

Offered as an alternative cleaning and emitter etching solution is anacidic aqueous mixture made up of tetramethylammonium hydroxide (TMAH),acetic acid, surfactants, complex-forming agents, fluoride, andperoxide. This solution makes possible stronger etching back of theemitters even at lower temperatures, that is, at approximately 40° C.

The emitter can be strongly etched-back by using an acidic etchingmixture consisting of nitric acid and hydrofluoric acid; that is, theincrease of the emitter layer resistance is, for example, 20 ohm/sq andmore at shorter contact time and lower temperature, such as, forexample, 60 s at up to 15° C. The etching rate of this solution is toohigh for a controlled moderate etching back of a flat emitter. Theremaining surface doping would be too small to form a low-ohmic contactwith silk-screened silver paste. This solution can be used to produce aselective emitter. In this type of emitter, subregions beneath the metalcontacts have a low layer resistance and a high surface concentration ofthe dopant as well as a high layer resistance between the metalcontacts.

Described as another possibility for emitter batch etching is a repeatedsequence consisting of chemical oxidation—for example, with nitricacid—and removal of the formed silicon oxide in dilute hydrofluoricacid. The etching back of the emitter in this way is very slow andrequires several process solutions.

The drawback of the available alkaline solutions for emitter etchingback is that the etching rate is too low. Long contact times and hightemperatures are necessary for an expedient, uniform, and moderateetching back of the emitter and an increase in the layer resistance inthe range between 5 and 15 ohm/sq.

Typical alkaline emitter etching solutions are based on ammonia orammonia derivatives and hydrogen peroxide. By way of example, referenceis made to the “SC1 solution” of the RCA cleaning developed forsemiconductor manufacture (W. Kern, “The Evolution of Silicon WaferCleaning Technology” in J. Electrochem. Soc., Vol. 137, No. 6, June1990, 1887-1891). Alkyl and hydroxyalkyl derivatives of ammonia offerthe advantage of a lower vapor pressure and hence less of a problem withemissions in comparison to ammonia. Further components, such ascomplexing agents, surfactants, and stabilizers, can also be employed(see, for example, WO A 2006/039090).

The drawback of these solutions is the low etching back of the emittersurface layer within the contact time available in standard processesfor solar cell manufacture, which usually is less than 1 min in acontinuous process unit.

Described in EP A 1 843 389 is a sequence consisting of repeatedchemical oxidation with subsequent dilute HF to remove the siliconoxide, so as to erode the uppermost highly doped emitter layers.Specified for chemical oxidation are: ozone, ozone/H₂O, O₃/H₂O/HF, H₂O₂,HNO₃, H₂SO₄, and NH₄OH at a temperature of between 20° C. and 90° C.This method is supposed to offer the advantage of a better degree ofcontrol of the emitter profile/phosphorus surface concentration createdduring diffusion with respect to oxidation. However, owing to chemicaloxidation under the given conditions, an oxide layer with a thickness ofonly approximately 1 nm is produced. Several repetitions of theoxidation/HF sequence would be needed to erode the highly doped layer.

Described in EP A EP 0 731 495 as cleaning solutions for semiconductorsin the framework of a modified RCA cleaning sequence are aqueous HFsolutions containing ozone (and surfactant for improvement of the ozonesolubility) or hydrogen peroxide.

An alternative possibility of avoiding the drawback of a high surfaceconcentration of dopant is offered by the development of the selectiveemitter. Thus, the production of a selective emitter via etching back ofan emitter, diffused by conventional processes, in the regions betweenthe metal contacts may be inferred from WO A 2009/013307. The regionsbeneath the metal contacts are protected by an etching barrierintroduced beforehand. In the first step, a mixture made up of nitricacid and hydrofluoric acid is used for etching back for controlledproduction of a porous silicon layer. The etching progress is readilyevident, because the porous silicon appears in various colors dependingon the layer thickness. In the second step, the porous silicon issubjected to wet-chemical oxidation. Specified as oxidants are HNO₃ andH₂SO₄. The removal of SiO₂ in dilute HF occurs subsequently.

A drawback of the mixed acid used is that it is difficult to control theformation of a homogenously porous Si layer by process engineering, sothat—and as a consequence of inhomogeneous etching back—a strong scatterof the emitter layer resistance values over the wafer surface results.

The effect of alkaline emitter etching solutions based on TMAH andhydrogen peroxide may be inferred from the literature reference J. J.Tool et al. “Almost 1% Absolute Efficiency Increase in mc—Solar CellManufacturing with Simple Adjustment to the Processing Sequence,”Proceedings 21^(st) European Photovoltaic Solar Conference, Dresden(2006).

A cleaning sequence consisting of repeated oxidation with HNO₃ and oxidedissolution in HF may be taken from S. Keipert-Colberg et al.,“Investigation and Development of Industrial Feasible Cleaning SequencesPrior to Silicon Nitride Deposition Enhancing Multicrystalline SiliconSolar Cell Efficiency,” Proceedings 24^(th) European Photovoltaic SolarEnergy Conference, Hamburg (2009).

The present invention is based on the problem of further developing amethod of the type mentioned in the beginning such that the drawbacks ofprior art can be avoided. At the same time, homogenous etching back ofthe surface region of the emitter should be possible. Furthermore, itshould be possible to employ process times and parameters that do notlead to a negative influence on the solar cell manufacturing process ina process line. It should also be possible to adjust precisely dopants,such as phosphorus surface concentrations. The blue sensitivity in solarcells should be improved.

In order to solve this problem, the invention provides essentially thatan oxidant-free alkaline etching solution, containing at least oneorganic moderator, is used for isotropic etching back of the emitter inits surface region, which has a dopant concentration of at least 10¹⁸atoms/cm³, particularly at least 10¹⁹ atoms/cm³. It is provided, inparticular, that the etching solution used is one in which the alkalinecomponent lies in the concentration range between 1 g/L (0.1 wt %) and50 g/L (5 wt %). In particular, it is provided that the concentrationrange of the alkaline component lies between 2 g/L (0.2 wt %) and 15 g/L(1.5 wt %). Furthermore, the etching solution used should be one whoseorganic moderator lies in the concentration range between 0.1 g/L (0.01wt %) and 5 g/L (0.5 wt %), in particular in the range between 0.2 g/L(0.02 wt %) and 1 g/L (0.1 wt %). Furthermore, it is provided, inparticular, that the contact time between etching solution and thesurface region is between 10 sec and 80 sec, preferably between 15 secand 60 sec, with the temperature of the etching solution preferablybeing in the range between 35° C. and 65° C., particularly between 45°C. and 60° C.

On the basis of the teaching of the invention, the surface region of theemitter, which exhibits planar homogeneous doping, that is, lateralhomogeneous doping, with at least a dopant concentration of greater than10¹⁸ atoms/cm³, is isotropically etched-back, with the topography of thepreviously textured surface being maintained. An emitter surface layerof constant thickness is consequently eroded. It is provided, inparticular, that, through the choice of contact time and temperature aswell as the concentration ranges of the alkaline component and of theorganic moderator, less than 10 nm is eroded, in particular between 3 nmand 7 nm, preferably in the range of about 5 nm.

The teaching of the invention avoids anisotropic etching withoutalteration of the topography of the surface, so that even fragilestructures projecting from the surface are retained.

Proposed in accordance with the invention is a method for amicroscopically homogeneous, wet-chemical, isotropic depth etching of ahighly doped emitter zone near the surface, with the greatest degree ofretention of the topographic details of the surface having a dopantconcentration of more than 10¹⁸ atoms/cm³ and a preferred back-etchingdepth of between 3 nm and 7 nm, in particular <5 nm, by using anoxidant-free alkaline etching solution containing at least one organicmoderator as etching solution.

Although the use of alkaline etching solutions containing organicmoderators is known, they are not known for isotropic etching back of ahighly doped surface region of an emitter of a silicon-based crystallinesolar cell.

Thus, a texturing and cleaning medium for surface treatment of wafersmay be inferred from DE A 10 2007 058 829. Proposed is an anisotropicetching for creation of surface roughness, so as to minimize lightreflections. Because of the anisotropy, the alkaline solutions used aresuitable only for texturing of monocrystalline silicon, with pyramidsbeing formed. Coming into consideration as etching medium are KOH, NaOH,THAH, or other inorganic etching media.

Provided as additives are aliphatic and aromatic carboxylic acids, aminocarboxylic acids, polyalcohols, EDTA, polyethylene sorbitan monolaurate,and alkly-substituted pyrocatechol. The concentration of the etchingmedium lies between 4 and 15 wt % and that of the additives between 1and 20 wt %. Provided as contact times are 10-30 min at a temperature ofgreater than 80° C. The silicon layer that is etched off lies in therange of between 5 and 10 μm. Use for etching back a highly dopedemitter surface region is not possible with a respective etchingsolution, because the emitter layer would be totally destroyed in thecase of conventional emitter thicknesses in the range of 350 nm.

The subject of DE A 10 2008 052 660 is a method for manufacturing asolar cell with two-stage doping. A selective emitter is supposed to beproduced by means of an inorganic mask. The mask is produced byapplication of a paste, which contains SiO₂ glass, and subsequentfusion. HF—HNO₃ is used for etching back. In this process, the mask ispartially or completely co-etched off. Employed for removal of theporous silicon resulting from the etching is an alkaline etchingsolution. The function of the alkaline etching solution involvesselectively etching off the porous silicon layer that was createdbeforehand in an upstream acidic etching step. An isotropic etching backin the range of 5 nm cannot be controlled using the respective measures.

Carried out in accordance with the invention, by contrast, is aone-step, chemically simple process, which, in comparison to thetechnically difficult process that, owing to the toxicity ofhydrofluoric acid, is also risky, is quite suitable economically.

An anisotropic etching by means of KOH with isopropanol, for example, isdescribed in GB A 2 209 245 for the creation of a three-dimensionalstructure. In this process, more highly doped regions are selectivelyremoved. Described is a multi-stage method for producing lateral etchedsteps in the surface, with the creation of a highly boron-doped surfacethat is resistant to the anisotropically acting etching medium.

A substrate that is 40 nm thick is thinned to 10 nm according to US A2004/0259324. Used as etching medium is an alkaline etching medium,which contains quaternary ammonium hydroxides, substituted amines,particularly TMAH in a concentration of between 10 wt % and 45 wt %. Theetching rate at 25° C. amounts to 7.5 nm/min. Provided as possiblefurther additives in the etching solution are anionic, cationic, andneutral surfactants, oxidants in the form of peroxide or persulfate, or,alternatively, reductants. Acids such as silicic acid can also beemployed to change the pH. The described method is aimed at substantialthinning of the substrate made of silicon, with the attainmentback-etching depths that, solely on account of the etching rate, do notlead to reproducibly adjustable back-etching depths for highly dopedemitters.

Etching solutions for undoped silicon, highly doped silicon, and siliconnitride are also known from US A 2005/0065050. The etching solution issupposed to enable the highest possible etching selectivity, that is,different etching rates for the respective substrates, to be achieved.Coming into consideration as etching medium are, for example, KOH orTMAH. Possible additives have the properties of being water soluble,non-volatile, non-flammable, with preferably ethylene glycol beingspecified. The etching method aims at the manifestation of a highselectivity of the etching rate of various dopant concentrations in thesilicon or at the boundary surfaces to adjacent layers, such as SiH orSiOx.

A selective dissolution of any unevenness in a thin-layer film is thesubject of US A 2010/0126961. The etching solution consists of a strongbase, such as TMAH, NaOH, or KOH, an etching moderator, an oxidant, suchas a persulfates, and a wetting agent. The contact time between theetching solution and the polysilicon thin-layer film that is beingplanarized lies between 0.5 min and 10 min at a temperature between 40°C. and 80° C. Projections on a size order of between 80 nm and 100 nmare supposed to be etched. Corresponding etching solutions are notsuitable for controlled used on highly doped surface regions of anemitter, particularly for a back-etching depth of <5 nm.

A selective etching of non-doped regions in silicon is described in U.S.Pat. No. 5,431,777. Coming into consideration as etching medium are KOH,NaOH, and TMAH in a concentration range of approximately 8 wt %. Asadditives, it is possible also to use aromatic alcohols and ethers,including substituted ones, Novolacs, and polyvinyl alcohol. The etchingrate ranges from a few nanometers to a few micrometers per minute, witha temperature range of between 45° C. and 104° C. be chosen.

EP A 2 302 701 relates to the texturing of a semiconductor substrate.Coming into consideration as etching medium are hydroxides and alkanolamines. The concentration range for use of KOH is given as 6 wt % by wayof example. Coming into consideration as additives are alkoxylatedglycols or glycol alkyl ethers, such as triethylene glycol anddiethylene glycol monomethyl ether. Additionally provided are chloridesand silicates. The etching erosion approaches 10 μm at temperatures ofbetween 80° C. and 100° C. The described mixtures are utilized foranisotropic texturing of a Si surface.

The subject of the invention is an alkaline etching solution containingat least one organic etching moderator. This solution makes possible anoptimal etching back of the emitter within short contact times, becausethe etching rate for silicon is higher than that of alkaline solutionsthat contain hydrogen peroxide as etching-moderating component.

Another advantage of the solution according to the invention is thatporous silicon, which can form in preceding process steps, is removed.

Surface-active substances (surfactants) can be employed as organicmoderators that inhibit the etching attack of the alkaline solutions.Because surfactant molecules contain a hydrophobic group and ahydrophilic group, both H-terminated (hydrophobic) and oxidized(hydrophilic) substrate surfaces are protected.

The advantage of organic moderators over hydrogen peroxide is that thesilicon surface is not completely “blocked.” Moderate and uniformsilicon dissolution is still not yet possible. Hydrogen peroxide resultsin oxidation of the silicon surface; the etching rate of silicon oxidein an alkaline solution is extremely low. Even at very lowconcentrations, hydrogen peroxide is effective above 0.1 wt %. Smallerconcentrations do not interfere and can also be present in the etchingsolutions according to the invention. Hence, it is insofar possible tospeak of an oxidant-free aqueous etching solution.

It has been found that many surface-active substances produce thedesired moderating effect on the etching operation. Differences in theadsorption strength of surfactants on the silicon surface and hence intheir ability to moderate the etching attack of the alkaline solutioncan be compensated for by adjusting the surfactant and alkalineconcentration as well as by adjusting the etching parameters temperatureand time to the desired etching rate or else to the desired increase inthe emitter layer resistance.

The solution according to the invention contains an alkaline componentand at least one organic etching moderator. Complexing agents andbuffering substances can be employed as further constituents.

Used as alkaline component is at least one substance from the group ofLiOH, NaOH, KOH, ammonium hydroxide, quaternary ammonium hydroxides,organic bases, and organic amines.

Quaternary ammonium hydroxides comprise tetraalkyl ammonium hydroxidesand substituted tetraalkyl ammonium hydroxides containing hydroxyl- andalkoxy-substituted alkyl groups, such as, for example, tetraalkylammonium hydroxide, trimethyl-2-hydroxyethylammonium hydroxide(choline), and triethyl(ethoxypropyl)ammonium hydroxide.

Examples of organic bases are pyrimidine and guanidine.

Organic amines comprise alkyl amines, polyalkylene amines,alkanolamines, cyclic N-substituted amines, and derivatives containingsubstituted alkyl groups.

Examples of alkyl amines are mono-, di-, and triethylamine anddodecyldimethylamine. An example of polyalkylene amines isdiethylenetriamine. Examples of alkanolamines are mono-, di-, andtriethanolamine and 2-(2-aminoethoxy)ethanol.

Examples of cyclic N-substituted amines are N-methylpyrrolidine,N-methylpiperidine, and N-ethylpyrrolidone.

The following substances can be employed as etching moderators.

anionic surfactants, such as sulfuric acid alkyl esters and the saltsthereof, alkyl carboxylic acids and the salts thereof, alkyl and alkylbenzene sulfonic acids and the salts thereof, mono- and diesters oforthophosphates, fluorinated carboxylic acids, and fluorinated sulfonicacids,

nonionic surfactants, such as polyalkylene glycol ethers (for example,fatty alcohol ethoxylates, fatty alcohol propoxylates), alkylpolyglucosides, saccharose esters, sorbitan fatty acid esters,N-methylglucamides, alkyl phenol ethoxylates and alkyl phenolpropoxylates, alkanol amides, alkyne diols, substituted alkynols,ethoxylated alkyne diols, and fluorinated alkyl alkoxylates,

amphoteric surfactants, such as alkyl betaines, amidoalkyl betaines,alkyl sulfobetaines, amidoalkyl sulfobetaines, alkyl amino oxides, alkylamidoalkyl amino oxides, and fluorinated amphoteric alkyl compounds,

cationic surfactants, such as amine ethoxylates, dialkyl dimethylammonium salts, and alkyl benzyldimethyl ammonium salts.

Examples of anionic surfactants are: sodium dodecyl hydrogen sulfate,sulfosuccinic acid dihexyl ester, sodium dodecylbenenesulfonate,ammonium lauryl sulfate, 2-ethylhexanol phosphoric acid ester, andperfluorooctanesulfonate.

Examples of nonionic surfactants are: tetraethylene glycol octyl ether,lauryl myristyl polyglycol ether, octylphenol ethoxylate, saccharosestearic acid ester, and 3,5-dimethylhexyn-3-ol.

Examples of amphoteric surfactants are: cocoamidopropyl betaine,dodecyldimethyl-amino oxide, octylimino dipropionate, andN-dodecyl-N,N-dimethylammonium propanesulfonate.

Examples of cationic surfactants are: oleylbis(2-hydroxyethyl)methylammonium chloride, dioctyldimethylammoniumchloride, and cocobenzyldimethylammonium chloride.

The emitter solution can additionally contain:

complexing agents for silicic acid (reaction product), such as, forexample, o-dihydroxybenzene and other hydroxyphenols as well as aromaticethers,

complex-forming agents or chelating agents for metal cations: aminessuch as EDTA, DTPA, di- and tricarboxylic acids, hydroxycarboxylic acids(for example, citric acid, tartaric acid), polyalcohols, such as, forexample, glycerin, sorbitol, and other sugars and sugar alcohols,phosphonic acids, and polyphosphates,

buffering substances, such as ammonium acetate and potassium hydrogenphthalate,

peroxides, such as hydrogen peroxide in very low concentrations (<0.1 wt%).

In addition, the invention is characterized by the use of one of thepreviously described etching solutions for etching back of the emitter,with a metal layer being deposited at least selectively onto the surfaceof the crystalline solar cell by chemical deposition orelectrodeposition of a nickel/silver or nickel/copper layer or byphysical vapor deposition methods after etching back of the emitter.When a vapor deposition method is used, a titanium/palladium/silverlayer, in particular, is deposited.

The field of application of the invention is the manufacture of solarcells made of silicon. Therefore, the invention is characterized by asolar cell, the emitter of which is etched-back by using measures thathave been previously described.

The invention will be discussed below on the basis of examples, fromwhich ensue further features, in themselves and/or in combination.

EXAMPLE 1

The etching effect of a hydrogen peroxide-containing alkaline solutionon the emitter of a solar cell was compared with the etching effect of asurfactant-containing solution. The hydrogen peroxide-containingsolution contained 1 to 6 weight percent caustic soda and approximately2 weight percent hydrogen peroxide. The temperature was in the range of49° C. to 55° C. The surfactant-containing solution contained 1.2 weightpercent NaOH and 0.1 wt % sodium dodecylbenzenesulfonate. Thetemperature of the solution was constant at 53° C.

Multicrystalline wafers, after diffusion and after chemical edgeisolation, including dissolution of phosphosilicate glass in dilutehydrofluoric acid, were etched in the alkaline solutions. The contacttimes and the resulting increase in the emitter layer resistance arecompiled in Table 1 and 2.

Emitter Layer Resistance

TABLE 1 Increase in the emitter layer resistance by treatment inhydrogen peroxide-containing solution. Contact Before After time etchingetching Delta Solution seconds ohm/sq ohm/sq ohm/sq NaOH 1 wt %, H₂O₂ 2wt % 60 48.1 48.7 0.6 300 49.3 50.5 1.2 NaOH 3 wt %, H₂O₂ 2 wt % 30 49.650.2 0.6 300 48.8 51.4 2.6

Emitter Layer Resistance

TABLE 2 Increase in the emitter layer resistance by treatment insurfactant-containing solution. Contact Before After time etchingetching Delta Solution seconds ohm/sq ohm/sq ohm/sq NaOH 1.2 wt %,sodium 25 54.0 58.9 4.9 dodecylbenzenesulfonate 50 55.3 68.1 12.8 0.1 wt% 75 55.9 78.6 22.7

It can be seen from the values that the etching rate of thesurfactant-containing solution is markedly higher than that of thehydrogen peroxide-containing solution.

EXAMPLE 2

Multicrystalline wafers, after diffusion, were etched in a hydrogenperoxide-containing or in a surfactant-containing solution afterchemical edge isolation, including dissolution of phosphosilicate glassin hydrofluoric acid. The wafers were then coated with a silicon nitrideantireflection layer in standard production processes and metallicizedin the silk-screen printing process. The electrical values of the cellsas well as the emitter layer resistance directly before a nitridecoating are presented in Table 3.

TABLE 3 Electrical values after emitter etching back in a hydrogenperoxide-containing or surfactant-containing solution. Jsc Voc [mA/ EtaR_(sheet) Emitter etching solution [mV] cm²] FF [%] Ω/□ NaOH, 1.2 wt %,hydrogen peroxide 0.609 34.07 0.772 16.02 64.0 0.3 wt % (20° C., contacttime 40 s) NaOH, 1.2 wt %, sodium 0.611 34.27 0.771 16.14 67.3dodecylbenzenesulfonate 0.1 wt % (50° C., contact time 25 s)

After a stronger etching back of the emitter, that is, at higher layerresistance, higher short-circuit current densities (Jsc) and higheropen-circuit voltages (Voc) are observed, resulting in a higherefficiency of the solar cells.

EXAMPLE 3

The etching effect of the solution according to the invention fromExample 1 and 2 was compared with the etching effect of a cleaningsolution consisting of 1 weight percent tetramethylammonium hydroxide(TMAH) and 1 weight percent hydrogen peroxide.

Used were multicrystalline wafers after diffusion and chemical edgeisolation, including dissolution of phosphosilicate glass in dilutehydrofluoric acid. After treatment in the emitter etching solutions wasfinished, further standard production processes were carried out.

The electrical values and the increase in the emitter layer resistanceare compiled in Table 4.

TABLE 4 Electrical values after emitter etching back in a solutionaccording to the invention and in an aqueous solution containing TMAHand hydrogen peroxide. Jsc Delta Voc [mA/ Eta R_(sheet) Emitter etchingsolution [mV] cm²] FF [%] Ω/□ NaOH, 1.2 wt %, sodium 0.603 34.24 0.76815.84 5.0 dodecylbenzenesulfonate 0.1 wt % (50° C., contact time 25 s)Tetramethylammonium hydroxide 0.601 34.03 0.769 15.77 1.5 1.0 wt %,hydrogen peroxide 1.0 wt % (60° C., contact time 60 s)

EXAMPLE 4

Multicrystalline wafers, after diffusion, with an emitter layerresistance of 48 ohm/sq, were treated in a vertical laboratory unitinitially in dilute hydrofluoric acid for 2 minutes, rinsed, andsubsequently etched in a solution of the following composition for 2minutes at 50° C.

KOH, 1.5 wt %

ammonium lauryl sulfate, 0.5 wt %

1,2-dihydroxybenzene (pyrocatechol), 0.1 wt %

The measured layer resistance after treatment in the etching solutionwas on average 53 ohm/sq.

EXAMPLE 5

In the same arrangement as in Example 2, a solution of the followingcomposition was used.

tetramethylammonium hydroxide, 1 wt %

ammonium lauryl sulfate, 0.1 wt %

diethlyenetriaminepentaacetic acid, 0.05 wt %

Monocrystalline wafers, after diffusion, with an emitter layer thicknessof 52 ohm/sq, were treated in dilute HF for 2 minutes, rinsed, andetched at 60° C. for 1 minute in the solution according to theinvention. The measured layer resistance was on average 58 ohm/sq. Theincrease in layer resistance was thus 6 ohm/sq.

EXAMPLE 6

An aqueous emitter etching solution with the following composition wasemployed:

diethylenetriamine, 3.5 wt %

dodecyldimethlyamine oxide, 0.1 wt %

ammonium chloride, 0.5 wt %

hydrogen peroxide, 0.06 wt %

Used were multicrystalline wafers after inline diffusion. After removalof phosphosilicate glass and after chemical edge isolation in ahorizontal unit, the emitter layer resistance was 53 ohm/sq.

After 1 minute of treatment time in a vertical laboratory unit at 50° C.in the solution according to the invention, the layer resistance was onaverage 56 ohm/sq.

1-19. (canceled)
 20. A method for the wet-chemical etching of a surfaceregion of a solar cell emitter in an alkaline etching solution,comprising: isotropic etching back the surface region using anoxidant-free alkaline etching solution comprising at least one organicmoderator, the surface region has a dopant concentration of at least10¹⁸ atoms/cm³.
 21. The method according to claim 20, wherein theoxidant-free alkaline etching solution has an alkaline component in aconcentration range between 1 g/L and 50 g/L.
 22. The method accordingto claim 20, wherein the oxidant-free alkaline etching solution has aconcentration range of the at least one organic moderator between 0.1g/L and 5 g/L.
 23. The method according to claim 20, wherein the step ofisotropic etching back comprises contacting the surface region with theoxidant-free alkaline etching solution for a time between greater thanor equal to 10 second and less than or equal to 180 seconds.
 24. Themethod according to claim 23, wherein the time is between greater thanor equal to 15 seconds and less than or equal to 16 seconds.
 25. Themethod according to claim 20, further comprising adjusting theoxidant-free alkaline etching solution to a temperature of between lessthan or equal to 35° C. and greater than or equal to 65° C. during theetching back.
 26. The method according to claim 21, wherein the alkalinecomponent has at least one component selected from the group consistingof LiOH, NaOH, KOH, ammonium hydroxide, quaternary ammonium hydroxides,tetraalkyl ammonium hydroxide, organic bases, and organic amines. 27.The method according to claim 20, wherein the organic moderatorcomprises at least one component selected from the group consisting ofsulfuric acid alkyl esters and the salts thereof, alkyl carboxylic acidsand the salts thereof, alkyl and alkyl benzene sulfonic acids and thesalts thereof, mono- and diesters of orthophosphates, fluorinatedcarboxylic acids, fluorinated sulfonic acids, polyalkylene glycolethers, such as fatty alcohol ethoxylates or fatty alcohol propoxylates,alkyl polyglucosides, saccharose esters, sorbitan fatty acid esters,N-methylglucamides, alkyl phenol ethoxylates, alkyl phenol propoxylates,alkanol amides, alkyne diols, substituted alkynols, ethoxylated alkynediols, fluorinated alkyl alkoxylates, alkyl betaines, amidoalkylbetaines, alkyl sulfobetaines, amidoalkyl sulfobetaines, alkyl aminooxides, alkyl amidoalkyl amino oxides, fluorinated amphoteric alkylcompounds, alkyl amine ethoxylates, dialkyl dimethyl ammonium salts, andalkyl benzyldimethyl ammonium salts.
 28. The method according to claim20, wherein the oxidant-free alkaline etching solution further comprisesat least one component selected from the group consisting of complexingagents, complexing agents for silicic acid, complexing agents for metalcations, chelating agents for metal cations, and buffering substances.29. The method according to claim 20, wherein the at least one organicmoderator is a surface-active substance or surfactant.
 30. The methodaccording to claim 20, wherein the at least one organic moderatorcomprises at least one component selected from the group consisting ofsodium dodecylbenzenesulfonate, saccharose stearic acid ester,dodecyldimethylamino oxide, and dioctyldimethylammonium chloride. 31.The method according to claim 20, wherein the step of isotropic etchingback the surface region comprises etching back the surface region to athickness between 0 and less than or equal to 10 nm.
 32. The methodaccording to claim 28, wherein the complexing agent is selected from thegroup consisting of hydroxyphenols, amines, hydroxycarboxylic acids,polyalcohols, phosphonic acids, and polyphosphates.
 33. The methodaccording to claim 20, wherein the step of isotropic etching back thesurface region comprises employing the oxidant-free alkaline etchingsolution in batch processing unit or continuous process unit.
 34. Themethod according claim 20, wherein the surface region comprises, as adopant, a material selected from the group consisting of phosphorus,arsenic, boron, gallium, and aluminum.
 35. A crystalline solar cell witha solar cell emitter that is etched-back according to the method ofclaim
 20. 36. A method for the wet-chemical etching of a surface regionof a solar cell emitter in an alkaline etching solution, comprising:isotropic etching back the surface region using an oxidant-free alkalineetching solution containing at least one organic moderator, the surfaceregion having a dopant concentration of at least 10¹⁸ atoms/cm³, whereinthe oxidant-free alkaline etching has an alkaline component with aconcentration range between 1 g/L and 50 g/L and the organic moderatorhas a concentration range that lies between 0.1 g/L and 5 g/L, whereinthe step of isotropic etching back comprises contacting the surfaceregion with the oxidant-free alkaline etching solution for a timebetween greater than or equal to 10 second and less than or equal to 180seconds; and adjusting the oxidant-free alkaline etching solution to atemperature of between less than or equal to 35° C. and greater than orequal to 65° C. during the etching back.
 37. The method according claim36, further comprising, after the step of isotropic etching back,applying a metal layer at least selectively onto the surface region by amethod selected from the group consisting of chemical deposition,electrodeposition, and physical vapor deposition.
 38. The methodaccording claim 37, wherein the metal layer comprises a layer selectedfrom the group consisting of nickel/silver layer, a nickel/copper layer,and a titanium/palladium/silver layer.
 39. A crystalline solar cell witha solar cell emitter that is etched-back according to the method ofclaim 36.